As the person skilled in the art knows, some groups of spacecraft, for example satellites, must be positioned with respect to each other with some accuracy in order to execute a mission collectively. This positioning must be effected at the start of the mission, for example when placing the satellites in orbit. However, it can also be effected during the mission, either to effect a partial or total reconfiguration of the group or to alleviate a technical failure (or equipment breakdown) affecting one or more of the spacecraft.
To provide for such positioning, it has been proposed to equip each spacecraft (or at least those most important to the mission) with a control device including, on the one hand, send/receive antennas, possibly complemented by receive antennas, installed on differently oriented faces of the spacecraft and responsible for sending/receiving radio-frequency (RF) signals, and, on the other hand, an “RF sensor” including first measuring means for estimating received signal path differences between antennas, and processing means for estimating the send directions of the signals that are sent by the other spacecraft of the group (generally referred to as “line of sight axes”), from the powers of the received signals.
Such control device can also include second measuring means responsible for estimating each difference between their spacecraft and one of the other spacecraft of the group from the signals received by the antennas and auxiliary signals sent by the other spacecraft of the group and representing the respective distances that separate them from their spacecraft. In this case, the processing means can determine the relative positions of the spacecraft of the group with respect to a chosen system of axes, on the basis of the estimated distances and the estimated line of sight axes.
Finally, if the control device includes analysis means, it can detect risks of collision between spacecraft from the relative positions determined, or even propose avoidance maneuvers of its spacecraft as a function of these relative positions, and possibly reconfigure the entire formation once the faults and the risks of collision have been overcome.
If the antennas are installed at well-chosen locations (for example to limit multiple paths) and the first measuring means use a robust method of alleviating ambiguity of the path difference measurements, it is possible to obtain path differences of the order of a few millimeters and therefore send directions having an accuracy of the order of one degree.
For the control device to be able to operate in all directions and thus to determine any relative positions, each spacecraft must be equipped on a number of faces with antenna triplets (one send/receive antenna and two receive antennas). Now, firstly, installing antennas on spacecraft is difficult, secondly, the large number of antennas complicates the control device, and, thirdly, alleviating ambiguity in respect of the path differences is a complex process that is difficult to make compatible with the robustness and the reaction time that are necessary for collision detection.
It is undoubtedly possible to use other techniques for estimating any relative positions of spacecraft, for example LIDAR or RADAR, but those techniques are costly and complicated, or difficult to install on spacecraft, notably on satellites.
It is equally possible to use a technique for estimating any relative positions based on relative GPS (Global Positioning System). However, this solution is not always adapted to formation flight missions either because the altitude of the mission is too high relative to the altitude of the GPS constellation or because supplementary or independent positioning means are required.
No known solution proving entirely satisfactory, an object of the invention is therefore to improve on the situation.